Processes of the emulsion polymerization of vinyl acetate (VAC) with ethylene, optionally with further comonomers, in aqueous phase for the purpose of preparing polymer dispersions are long established. In order to achieve colloidal stability in the dispersions produced, the polymerization process uses water-soluble polymers such as, for example, polyvinyl alcohols, cellulose derivatives and polyvinylpyrrolidones as protective colloids or low molecular mass surface-active substances, referred to as emulsifiers or surfactants. Also long and well established, for the purpose of achieving good colloidal stability and/or of regulating the particle sizes, is the use of combinations of polyvinyl alcohols and emulsifiers during the polymerization.
DE-A 1595402 describes a process in which the rate of incorporation of ethylene into vinyl acetate-ethylene copolymers is improved by the inclusion of at least 75% of the vinyl acetate monomer in the initial charge. The polymerization takes place in the presence of nonionic emulsifiers. DE-A 2112769, for the purpose of ensuring a uniform polymerization profile, recommends, for the copolymerization of vinyl esters with ethylene, using a redox initiator system, including the reducing agent with the vinyl ester fraction, the ethylene and the dispersants fully in the initial charge, and metering in only the oxidizing component. U.S. Pat. No. 3,644,262 describes a process for preparing VAE dispersions that leads to improved rates of ethylene incorporation. To achieve this, the vinyl acetate fraction and the emulsifier and/or protective colloid fraction are metered in full or included only in very small fractions in the initial charge. This process allows up to about 20% of ethylene to be incorporated in the copolymer at low pressures and temperatures. A disadvantage is that none of the products produced ensures adequate adhesion between polymer substrates and paper or cardboard.
The aqueous polymer dispersions based on vinyl acetate and ethylene (VAE dispersions) that are produced in this way can be used with advantage as binders, more particularly for adhesives, for paints or for bonding fibre to nonwoven web. Over the course of the years, the requirements imposed on the profile of properties of such VAE dispersions have become increasingly more specific. For example, in the context of the adhesive utility, a trend has become evident to impose specific requirements on the bonding of materials with low surface energy, as for example when bonding polymer films to cellulosic materials, especially to paper and cardboard. Customary VAE dispersions are unsuitable for this purpose, providing inadequate adhesion on the part of the adhesively bonded assembly. A variety of efforts have therefore been undertaken to provide VAE dispersions with improved adhesion.
U.S. Pat. No. 3,708,388 describes adhesives based on vinyl acetate-ethylene copolymers that are stabilized exclusively with alkylphenol ethoxylate emulsifier(s) or with protective colloid and such emulsifiers, that have a defined intrinsic viscosity and that can be used for the lamination of films. In the preparation process, at least 75% of the vinyl acetate (in the examples 96.5% to 100%) is included in the initial charge. For the purpose of improving the mechanical strength at high temperatures, the use of crosslinkers is advised.
EP 1212383 B1 describes a process for producing adhesives featuring improved adhesion. It operates by admixing aqueous dispersions, produced by emulsion polymerization, after the end of the polymerization, subsequently, with 0.3% to 3.0% by weight of one or more emulsifiers, based on the total weight of the dispersion. The specification teaches that an improvement in adhesion occurs only on subsequent addition of the emulsifier, whereas the use of emulsifier during the polymerization is said not to result in any improvement in adhesion. Disadvantageous features of this procedure are that a balanced trade-off between adhesion and cohesion is not provided, the adhesion to polystyrene and polyester films is much too low, and the setting rate of the dispersions is too low.
EP 890625 B1 describes an improved vinyl acetate-ethylene (VAE) latex polymer, and a process for preparing it, which is said to be suitable as an adhesive for packaging and for difficult-to-bond surfaces such as polyethylene, polyester, metallized polyester and oriented polypropylene. The polymer is characterized by means of particular temperature-dependent values of the storage modulus at a defined test frequency. For its preparation, the use of regulators is advised, and, in the polymerization process, less than 15% of the vinyl acetate is included in the initial reactor charge, and the vinyl acetate content is maintained during the polymerization at less than 5%. Polymer dispersions obtainable in this way in fact possess a high adhesion of difficult-to-bond polymer films to paper or textile. The disadvantage of the polymers thus produced, however, is that the cohesion of the adhesive bond is substantially too low, since the profile of requirements for the storage modulus does not allow sufficiently high cohesion. Moreover, alkylphenol ethoxylate having 4 to 100 ethylene oxide units is used as an emulsifier. Such emulsifiers, however, are extremely problematic on account of their environmental and health burden.
EP 385734 B1 describes a VAE copolymer and a polymerization process for its preparation, yielding an aqueous polymer dispersion having a solids content of 65% to 75%. The VAE copolymer is said to be suitable as an adhesive for a wide variety of substrates, with a high setting rate. It is prepared using a dispersant mixture comprising partially hydrolysed polyvinyl alcohol and a nonionic polyoxyethylene emulsifier with a defined HLB. The polymerization process is characterized in that 40% to 90% by weight of the vinyl acetate are incorporated by emulsification into an aqueous solution comprising the entire polyvinyl alcohol, the entire emulsifier and the entire reducing agent, thereby forming a stable emulsion, and subsequently, following injection of ethylene, the remaining 10% to 60% by weight of the vinyl acetate is metered in. A particular feature of the operation that is emphasized is the use of 40% to 90% of the vinyl acetate in the preliminary emulsion step, the remainder being metered in during the polymerization. The process does not lead to a carefully weighed balance between adhesion and cohesion; the adhesion to polystyrene and polyester films is too low.
U.S. Pat. No. 4,267,090 describes a process for preparing VAE dispersions, with the objective of improving the wettability of polyvinyl chloride substrate and the setting rate, with continued high or improved cohesion, through the use of defined amounts of emulsifier (not less than 1% and not more than 2%). The polymerization process is further characterized in that the vinyl acetate and the emulsifier fraction are part included in the initial charge and part metered in. The protective colloid fraction and the oxidizing agent in U.S. Pat. No. 4,267,090 are included fully in the initial charge. Polymers in accordance with U.S. Pat. No. 4,267,090 do not result in the desired, carefully weighed balance of adhesion and cohesion, and provide inadequate adhesion values for polystyrene and polyester films. To obtain the requisite cohesion, determined by a creep test, the “inherent viscosity” is supposed to have a value of at least 1.9 dl/g (with the details in U.S. Pat. No. 4,267,090, corresponding to an intrinsic viscosity of 1.95 dl/g) for the polymer fully hydrolysed in concentrated acetic acid. For the unhydrolysed VAE polymer with an ethylene fraction, according to the examples given, of 13.6% by weight, based on total monomer, the molar mass is higher by a factor of 1.73 than for the fully hydrolysed polymer. According to the known relationship between viscosity average of the molar mass M and intrinsic viscosity [η] (H. G. Elias, Polymere, Von Monomeren and Makromolekülen zu Werkstoffen; Hüthig & Wepf Verlag, 1996, page 202 ff): [η]=a Mα (where α=0.764), accordingly, the intrinsic viscosity of the unhydrolysed polymer is higher by a factor of 1.52 and is therefore at least [η]=2.96 dl/g (corresponding to a K value for the polymer of at least 125).
U.S. Pat. No. 5,571,860 proposes improving the setting rate, heat stability and adhesion of adhesives based on vinyl acetate-ethylene copolymers by copolymerizing them with N-vinyl-formamide or N-vinylpyrrolidone. The heat stability is further improved by incorporation of 0.5% to 10% by weight of crosslinkable monomer, more particularly glycidyl methacrylate (GMA). The polymerization process used provides for 70% of the vinyl acetate employed to be included, together with 90% of the dispersants employed, in the initial charge to the reactor, with the remainder metered in. A dispersant used is polyvinyl alcohol, optionally in a mixture with emulsifier. A disadvantage is the copolymerization of N-vinylformamide or N-vinylpyrrolidone, as it requires relatively high polymerization temperatures and hence relatively high pressures, and hinders the incorporation of ethylene.
EP 279384 B1 describes the preparation of vinyl acetate-ethylene copolymer dispersions with suitability as packaging adhesives. It was found that, when using a combination of low molecular mass polyvinyl alcohol and nonionic emulsifier to stabilize the dispersion, the setting rate of the adhesive improves, even at high solids content. In the polymerization, at least part of the low molecular mass polyvinyl alcohol is included in the initial charge, and all of the emulsifier fraction is included in the initial charge. A disadvantage affecting the product obtained by the process is the much-too-low adhesion between cellulosic substrate and polystyrene or polyester films.
An object of U.S. Pat. No. 3,769,151 was to provide a VAE adhesive featuring improved adhesion to polymer films, more particularly to vinyl polymers. A process is described of seed emulsion polymerization of vinyl acetate, ethylene and up to 1% by weight of unsaturated acids, using a mixture of partially hydrolysed and fully hydrolysed polyvinyl alcohols, with only partially hydrolysed polyvinyl alcohol being used to prepare the seed, and with a blend with polyvinyl alcohol having a higher degree of hydrolysis being used in the seed polymerization. Also described is the possibility of additionally using other additives during the polymerization without substantially affecting the adhesive properties. Examples given of such additives which are said to have no effect on the adhesive and cohesive properties of the VAE dispersions are nonionic, cationic or anionic emulsifiers. Aqueous polymers in accordance with U.S. Pat. No. 3,769,151 possess good cohesion, but the adhesion, especially to polystyrene and polyester, is much too low and thus entirely inadequate.
GB-A 1546275 describes a process for obtaining VAE dispersions which in their pressure-sensitive adhesive utility display a good balance in terms of tack, cohesion and adhesion. As shown by the inventive examples, which use exclusively hydroxyethyl-cellulose as protective colloid, and the comparative examples, achieving good adhesion requires the use of chain transfer agents and a protective colloid fraction of not more than 1% by weight, based on total monomer. GB-A 1546275, moreover, requires intrinsic viscosities of between 0.6 and 1.0 dl/g. The products obtained in this way do possess good adhesion properties, but have the disadvantages of inadequate cohesion and an inadequate setting rate.